This species is poorly known, with only 10 localities (representing 10 locations) across a relatively wide area, although not many appropriate surveys have been carried out. Since this species is restricted to streams within natural forests, there is estimated to be less than 2,000 km2 habitat remaining and this is used as a proxy for the area of occupancy (AOO). These streams are undergoing a decline in habitat quality, especially from siltation, and the species' AOO is declining. Further research is needed to determine how restricted this species is within the forested stream habitats of its range and to what extent its main threats (habitat loss and degradation and trapping) are affecting populations, and such research certainly could elevate its threat status. For these reasons, the species is listed as Vulnerable under B2ab(ii,iii).

This species is endemic to Madagascar. It occurs in the eastern humid forest and central highlands. This species has only been recorded from a few localities in Madagascar (10 sites listed in Benstead and Olson 2003), including the Antsanpandrano Forestry Station on the Ankarata Massif; a site 15 km north of Antanifotsy village (Andringitra); a site 10 km north of Andekaleka, one specimen was caught in an eel trap at Ranomafana Est, 60 km east of Fianarantsoa; and a site 35 km south of Antsirabe. Its amphibious lifestyle is an impediment to the study of its distribution, which is poorly known. The altitudinal range is 450-2,000 m asl (Benstead et al. 2001). There have not been many appropriate surveys in the intervening areas between the known localities. Although it may be present in appropriate habitat, it is of note that several apparently suitable sites in eastern Madagascar do not support populations (Nicoll and Rathbun 1990)

This species is only known from a few sites (10 are listed in Benstead and Olson 2003). Only two sites (Antsanpandrano and Ramonafana) have had repeated survey work. It appears to be declining in some places (Benstead and Olson 2003).

This species is a semi-aquatic carnivore, occurring along streams and rivers. The main habitat requirements are permanent, clean and fast flowing water. This species is thought to feed on freshwater crayfish, aquatic insect larvae and small crustaceans. However, it has once been found in streams within a pine plantations (which potentially keep siltation down similar to native vegetation), although this does not seem to be the case in other areas within its range (S.M. Goodman pers. comm.). For more on ecology see Benstead et al. 2001, Benstead and Olson 2003. This is the largest species in the subfamily Oryzorictinae and its body size probably an adaptation to its aquatic lifestyle (Stephenson 1994).

This species apparently requires clean and fast-flowing water and is therefore vulnerable to siltation and soil erosion caused by deforestation. Agricultural expansion is fragmenting the upland forests, thereby isolating fast-flowing riverine habitat. Previously, collection and removal of the plant Aponogeton, which L. mergulus is closely associated with, was thought to cause an adverse reaction from the population (Nowak 1999); however, this has since been refuted (Benstead and Olson 2003). Accidental capture in fish traps has also been recorded. The species can no longer be found, or is rare, in sites previously known to support populations (e.g. Andekelaka and Antsanpandrano; Benstead and Olson 2003) suggesting threats such as deforestation habitat degradation and drowning in eel or crayfish traps may be causing a population decline.

This species as known from in or around two protected areas, Ranomafana National Park and the Andringitra National Park (Nicoll and Langrand 1989) and is also reported in the new Nosy Volo Reserve in the east (Nicoll and Atsifandrihamanana 2014).

The vulnerability of benthic prey communities to sedimentation implies that prevention of erosion and sedimentation is of paramount importance for conservation of this species. New and existing sites for the species must be protected from the effects of sedimentation wherever possible, either by inclusion of forested catchments in the protected areas network or by other means (e.g., effective terracing of agricultural fields and maintenance of vegetated riparian zones; Benstead et al. 2001).